it had
gained half an oscillation, i. e. when the large one was moving forward,
the small one would be coming back.

"The big one moves the fastest," said Rollo.

"Not much," said Mary.

"No," said Rollo, "not much."

"And I don't think it is owing to the difference in the bigness of the
stones."

"What else can it be?" said Rollo. "They are exactly alike in all other
respects."

"Not exactly," said Mary. "We have made them as nearly alike as we
could, but not exactly. There may be a good many little differences that
we do not observe. But if the size of the stone would cause any
difference in the vibrations, I should think it would make a much
greater difference, for one is twice as big as the other."

"Let us try a _very_ big stone," said Rollo.

"Well," said Mary.

So Rollo got a stone as large as an orange, which was as heavy a one as
Mary thought the thread would hold; and Mary suspended that from the
branch of the tree, and then swung it in company with the two others.
They all went very nearly together at first, though there was evidently
a slight difference, which, in a short time, separated the oscillations,
so that the stones did not keep together; while yet they each swung back
and forth, in nearly the same time. Rollo and Mary both concluded, from
the result of this experiment, that the size of the vibrating body did
not perceptibly affect the rapidity of the vibrations.

"Now," said Mary, "we will try different lengths of string."

So she began to look over Rollo's pebbles, to find two as nearly as
possible alike.

"The pebble stones must be of the same size, this time, for we want the
two pendulums to be alike in all respects, except the length of the
string, for that is the circumstance which we are now going to consider.
We will have one string twice as long as the other."

Mary found two pebbles very nearly equal in size, and similar in shape.
She tied them to two strings, making one string twice as long as the
other. She suspended them as before, and then, taking hold of one with
one hand, and the other with the other, she drew them out to the same
distance on one side, and let them go. The short one began at once to
swing back and forth very quick, while the other followed quite slowly.

"That makes a difference," said Rollo, clapping his hands.

"It goes twice as fast," said Mary.

"More than twice as fast," said Rollo, "I think."

"Let us see," said Mary.

They set them vibra